Wells are generally drilled into the ground or ocean bed to recover natural deposits of oil and gas, as well as other desirable materials that are trapped in subterranean formations. Such wells are drilled into a formation using a drill bit attached to the lower end of a drill string. Drilling fluid is pumped from the wellsite surface down through the drill string to the drill bit. The drilling fluid lubricates and cools the bit, and may additionally carry drill cuttings from the wellbore back to the well site surface.
Differential pressure sticking can cause excessive friction between portions of the drill string and a sidewall of the wellbore, which can immobilize the drill sting within the wellbore. Differential pressure sticking is caused by the difference between the pressure of the formation and the hydrostatic pressure within the wellbore. Formations that are more permeable can present higher risks of differential sticking, because the higher permeability can increase “filter cake” deposition on the sidewall of the wellbore. That is, the differential between the higher wellbore pressure and the lower formation pressure urges drilling fluid into the formation, and a higher permeability of the formation permits more of the liquid portion of the drilling fluid to invade the formation, which results in more of the solids portion of the drilling fluid to build up on the sidewall of the wellbore. As the filter cake thickness increases, the risk of differential pressure sticking also increases, because thicker filter cake exposes more of the drill string circumference to the lower formation pressure, thus increasing the net force of the hydrostatic pressure urging the drill string into the sidewall of the wellbore.
The increasing net force of the hydrostatic pressure urging the drill string into the sidewall of the wellbore also increases friction between the drill string and the sidewall of the wellbore. When the friction exceeds the maximum pulling power of the rig, the drill string cannot be pulled free, such that the bottom hole assembly (BHA) at the bottom of the drill string is deemed “lost-in-hole” (LIH). Differential pressure sticking accounts for large monetary losses to the oil and gas industry, whether in the form of LIH BHAs, stuck pipe/BHA recovery efforts, or other consequential equipment recovery and/or wellbore reconstruction actions, including lost well sections to be re-drilled.